Stents are generally cylindrical-shaped devices that are radially expandable to hold open a segment of a vessel or other anatomical lumen after implantation into the body lumen.
Various types of stents are in use, including expandable and self-expanding stents. Expandable stents generally are conveyed to the area to be treated on balloon catheters or other expandable devices. For insertion into the body, the stent is positioned in a compressed configuration on the delivery device. For example, the stent may be crimped onto a balloon that is folded or otherwise wrapped about the distal portion of a catheter body that is part of the delivery device. After the stent is positioned across the lesion, it is expanded by the delivery device, causing the diameter of the stent to expand. For a self-expanding stent, commonly a sheath is retracted, allowing the stent to expand.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications, including intravascular angioplasty. For example, a balloon catheter device is inflated during percutaneous transluminal coronary angioplasty (PTCA) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. When inflated, the pressurized balloon exerts a compressive force on the lesion, thereby increasing
The inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels restenose.
To prevent restenosis, stents, constructed of metals or polymers, are implanted within the vessel to maintain lumen size. The stent is sufficiently longitudinally flexible so that it can be transported through the cardiovascular system. In addition, the stent requires sufficient radial strength to act as a scaffold and support the lumen wall in a circular, open configuration. Configurations of stents include a helical coil, and a cylindrical sleeve defined by a mesh, which may be supported by struts or a series of rings fastened together by struts.
Stent insertion may cause undesirable reactions such as inflammation resulting from a foreign body reaction, infection, thrombosis, and proliferation of cell growth that occludes the passageway. Stents with polymer coatings have been used to deliver drugs or other therapeutic agents at the site of the stent that may assist in preventing these conditions. Another approach to this problem is to use biodegradable stents composed of polymers that, over a defined period of time, are removed from the body. Such temporary implants remain in place during healing at the treatment site, but then disappear, thereby minimizing many of the deleterious effects of long term implants such as inflammation, cellular proliferation and thrombosis.
Another parameter to be considered in stent design is diameter recoil, the tendency of the stent to revert toward its compressed diameter following expansion. Diameter recoil or constriction is due primarily to the elastic properties of the material comprising the stent, and is generally greater for polymeric stents than for those comprising metals such as stainless steel. Diameter recoil may cause the stent to partially block blood flow through the vessel, or to become dislodged from the treatment site.
It would be desirable, therefore, to provide an implantable polymeric stent that retains the longitudinal flexibility needed for efficient delivery and the radial strength to support the vessel wall, but also exhibits minimal diameter recoil upon expansion of the stent, and may additionally be biodegradable. Such a stent would overcome many of the limitations and disadvantages inherent in the devices described above.